Conventional pacemakers and ICDs require manual programming of numerous programmable parameters, including but not limited to: atrial sensitivity, ventricular sensitivity, post-ventricular atrial refractory period (PVARP), post-ventricular atrial blanking period (PVAB), and other parameters such as ventricular refractory period, ventricular output, atrial output, choice of pacing mode, upper rate limit, base rate, sleep rate, sensor slope, sensor threshold, and so forth. The programming of these parameters can be inaccurate and time consuming, and requires highly-skilled medical expertise to accomplish.
An added complication to the process of manually programming atrial or ventricular sensitivity is that signal amplitudes observed on EGM monitors in a clinical setting do not correlate well with the signal amplitudes sensed by the implanted pacemaker or ICD after the signal has been processed by internal circuitry which includes various filters and amplifiers. Attempts to automate programming of sensitivity have not been completely successful, in part because of the natural fluctuation in the amplitudes of myopotentials and the incidence of noise associated with skeletal muscle depolarizations in unipolar systems and fluctuations in the amplitude of the intrinsic beats with marked differences between the sinus P wave and conducted ORS complex with respect to ectopic beats which cannot be assessed at the time of implantation or follow-up because they are not present at those times in both the unipolar and bipolar sensing configurations.
Selecting appropriate atrial sensitivity can be particularly challenging due to the low amplitude of atrial events, the small differences between sinus atrial event amplitudes and ectopic atrial event amplitudes, and the desire to sense and accurately detect low level signals associated with atrial fibrillation, flutter or ectopic foci.
The importance of correctly programming atrial sensitivity, however, cannot be over emphasized. An incorrect high atrial sensitivity would predispose to over-sensing and inappropriate mode switching. An overly low atrial sensitivity would prevent the correct detection of atrial fibrillation and would result in inappropriate ventricular pacing during atrial fibrillation associated with sensing and triggering on the intermittent atrial signals of sufficient amplitude, with serious hemodynamic consequences. In addition, under-sensing of premature atrial contractions (PACs) and premature ventricular contractions (PVCs) can result in arrhythmia induction induced by competition.
The problem of automatically and accurately sensing P-waves and R-waves is even more pronounced when using an “A-V cross-chamber” electrode configuration, that is, an electrode configuration in which the stimulation device senses cardiac signals between an atrial tip electrode and a ventricular tip electrode, and stimulates each chamber in a unipolar fashion from the respective electrode to the housing (i.e., typically referred to as the case electrode). When such electrodes are implanted, various electrode sensing configurations are possible, e.g., atrial unipolar (A tip-case); ventricular unipolar (V tip-case): atrial-ventricular cross-chamber (A tip-V tip); ventricular unipolar ring (V ring-to-case), atrial unipolar ring (A ring-to-case), atrial bipolar (A tip-ring) or ventricular bipolar (V tip-ring).
While unipolar sensing configurations are more susceptible to extraneous noise, bipolar sensing configurations are also susceptible to problems of oversensing depending on electrode position and spacing.
Regardless of the cardiac event being sensed, and regardless of the electrode configuration being used, there is a need for an implantable device which is able to readily and reliably sense P-waves, R-waves, premature atrial contractions (PACs), and premature ventricular contractions (PVCs). The implantable device, if it is to perform its intended function, must correctly detect a sinus atrial depolarization (P-wave), a sinus-induced ventricular depolarization (R-wave), and it must not incorrectly detect a PVC or a PAC as a sinus P-wave or R-wave, or vice versa, and thus inappropriately adjust pacing parameters.
While it is well known that various blanking schemes may be used to block or blank out unwanted inappropriate physiologic signals such as far-field signals or retrograde P-waves by using different blanking intervals (i.e., PVARP, automatic PVARP extension, PVAB, etc.), and thereby prevent these far-field signals or retrograde P-waves from being falsely sensed as P-waves, such blanking schemes (based solely on timing considerations) have proven less than satisfactory because legitimate (anterograde) P-waves and PACs that need to be sensed, may and do occur during these blanking intervals.
A number of attempts have been made previously to provide accurate sensing and detection of cardiac events by incorporating an additional physiological signal to verify signal detection by the primary electrogram (EGM) sensing of the electrical events of the heart. An important limitation of this approach of adding a second physiological sensor is the added hardware required to implant in the patient and the additional circuitry required to interpret more than one physiological signal and relate them.
Other attempts to improve signal detection have focused on alternative approaches in processing the EGM signal by modifying the detection circuitry, thus avoiding additional sensor implantation. Various schemes have been proposed such as using two sense amplifiers receiving the same signal but possessing different sensitivity settings that are adjusted in; tandem, two comparators with different threshold detection levels, or more than one signal processing parameter (such as amplitude and slew rate).
A major limitation of these methods is that in order to make automatic adjustments to sensitivity, a stable rhythm is required. In essence, during sensitivity adjustments, the normal function of the sensing circuitry is interrupted momentarily. This circuitry is integral to the timing operations of the pacemaker or ICD which is why it is possible to first verify rhythm stability prior to initiating any threshold or sensitivity testing or adjustment. Thus, frequent premature beats or irregular rates might inhibit automatic sensing threshold tests and sensitivity adjustment and lead to prolonged periods of inappropriate sensing and even delivery of electrotherapies with adverse effects based on erroneous detections.
A further limitation of conventional devices is that the primary sensing circuitry is automatically adjusted such that only a target event of interest is detected, for example P-waves but not PACs. Such exclusive sensing eliminates the tracking of certain non-sinus events that are important to detect for proper pacemaker function and are of diagnostic interest to a medical practitioner.
Therefore, there is still an unsatisfied need to automatically monitor sensing thresholds of various cardiac events, such as P-waves and PACs, R-waves and PVCs, and to accurately detect and discriminate these events without interruption of the normal operation of the pacemaker. This need becomes particularly acute when sensing between inter-chamber electrodes, e.g., when sensing using an A-V cross-chamber electrode configuration.